Mining machines of the aforedescribed type have been provided heretofore, especially for excavating narrow seams by attacking the coal face in a relatively wide cut which corresponds, for example, to at least a portion of the length of the two stretches. The machine is advanced in the direction of the coal face, i.e. transversely to the two stretches, and the coal is excavated by picks or the like of the tools and the excavated material is displaced along the conveying trough to one or the other end of the machine, e.g. into a gallery, tunnel or the like from which the conveyed material can be carried off.
At least one drive unit can be provided for one of the reversing wheels around which the chain passes and can comprise an electric motor and a transmission. The drive station provides a certain tension at least in the excavating stretch for the chain.
In conventional long-wall mining, socalled walking units are commonly provided such that the supported or propped parts were braced upon those riding on the floor of the cut and the advance of the excavating unit is effected in a step-like manner. The trailing part of the machine is thus drawn along after the leading part has been moved forwardly. The resulting excavation, which ultimately deposits the excavated coal in a gallery alongside the cut, requires step like advance of the leading side of the machines at a rate which will follow the plowing of the coal from the seam so that the plows or tools will not have nonworking passes.
The conveyor operates like a flight conveyor, utilizing the same tools to shift the excavated material which falls into the conveying trough toward the end of the machine and into a gallery.
Machines of this type are known from, for example, DE 35 45 302 C2 and DE 40 04 488 and can have various configurations. They have been found to be largely satisfactory and to provide significant rates of removal of the material, i.e. mined coal per unit time. However, in spite of the success of such machines, it has become desirable to further improve the mining efficiency.
One of the problems with conventional systems appears to be the frictional loss which arises in the conveying and excavating troughs and the work in advancing the excavating part and the drawing of the trailing part of the machine thereafter.
Furthermore, the conventional mining machine must have a fairly large cross section and span a substantial length, all of which have posed problems with respect to stability of the machine. In the conventional systems with such machines, undercutting of the coal itself to allow the coal to then drop into the conveying trough was limited and by and large the tools had to be dimensioned to plow coal from the full thickness or height of the coal seam.